1. Field of the Invention
Oxide-dispersion-hardened nickel-based super-alloys which, thanks to their outstanding mechanical properties at high temperatures, are used in the construction of thermal machine components subjected to high thermal and mechanical loads. Preferred field: blading materials for gas turbines.
The invention relates to the further development and extension of the field of application of relatively brittle oxide-dispersion-hardened superalloys, which have high hot strength but are difficult to shape and which, as a monolithic preliminary material with directional coarse grained columnar structure of the crystallites, can primarily only be manufactured in limited cross-sectional dimensions.
In particular, it relates to a method of manufacturing a workpiece of any cross-sectional dimensions from an oxide-dispersion-hardened nickel-based superalloy with directional coarse columnar crystals set with their longitudinal axis at right angles to the cross-sectional area.
2. Discussion of Background
Oxide-dispersion-hardened superalloys based on nickel have attractive high temperature mechanical properties and permit the working medium temperatures of thermal machines to be increased by a further 100.degree. to 150.degree. C. relative to non-dispersion-hardened nickel-based superalloys. This is highly desirable from the point of view of increasing the efficiency of energy conversion. In order to exploit these alloys fully, however, the workpieces manufactured from them must exist in the coarse crystalline condition. In the case of a shaft-type component with a pronounced longitudinal axis, this means that the material must be available in the form of longitudinally directed columnar crystals. It is only by this means that the high creep strengths can be achieved at extremely high duty temperatures.
Oxide-dispersion-hardened superalloys are manufactured powder-metallurgically by mechanical alloying. The previously compressed material is, generally speaking, processed by extrusion to produce a dense, pore-free semi-finished product. Because of its high hot strength, the extrusion cross-section obtainable are limited to certain dimensions because of the limited forces of the extrusion presses. In order to achieve longitudinally directed columnar crystals, the semi-finished product must be subjected to a zone heat treatment process (directional recrystallization). This demands certain heating and cooling conditions over the whole of the cross-section. The cross-section of the semi-finished product to be treated is therefore again limited for thermodynamic reasons (temperature gradient). The largest rectangular cross-sections attainable on the basis of the thermal conductivity of the material are about 35 mm.times.110 mm. A prismatic semi-finished product of 32 mm.times.105 mm is currently obtainable commercially. In consequence, oxide-dispersion-hardened superalloys can, at the moment, only be employed for limited blading sizes.
In the manufacture of industrial gas turbines of high power, the blading dimensions required already exceed the cross-sections of the commercially obtainable and technically feasible semi-finished products in oxide-dispersion-hardened alloys. In consequence, the use of these alloys is currently excluded precisely where their employment would be most promising.
It has already been proposed that parts in an oxide-dispersion-hardened superalloy in fine-grained condition should be combined by diffusion bonding to form larger bodies and that these should subsequently be converted to the coarse-grained condition by recrystallization heat treatment (see US-A-3 758 741). This method fails, however, in the case of the zone heat treatment of large cross-sections. Because of a lack of sufficiently high temperature gradients, the parts located within the workpiece cannot be forced to recrystallize into coarse-grained columnar crystals. As a result, the creep strength drops to a fraction of the optimum value. The workpiece is useless for the intended purpose.
Hot shaping of sheets of coarse-grained oxide-dispersion-hardened materials which are difficult to deform by hot isostatic pressing and, alternatively, simultaneous diffusion bonding of several sheets or sheet ends is known (see, for example, EP-A-0 192 105). This patent concerns limited cross-sections and dimensions in which the problems mentioned above do not, as a rule, appear. Furthermore, the relevant bonding zones are not generally subjected to high loads in operation and, in consequence, the appearance of any fine-grained zones which may occur has no serious effect on the functional capability of the workpiece.
There is a large requirement to extend the field of application of oxide-dispersion-hardened materials in the construction of thermal machines. The designer should not be restricted by any sort of limitations with respect to the dimensions of the workpieces; this will become increasingly important in future because of the steadily increasing size of the machine units (industrial gas turbines, for example).